1. Field of the Invention
The present invention relates to a golf ball covered with a thermosetting polyurethane cover that enables the golf ball to maintain a high repulsion property even if the ball is left under a severe condition such as a rainy weather or a very hot weather under a burning sun.
2. Description of the Related Art
Ionomer resin covers are mainly used for golf ball covers because they have superior durability. Golf balls with such ionomer resin covers, however, give golfers a larger impact upon a shot than golf balls with Balata covers and hence are likely to impart the golfers with an inferior shot feeling.
In attempt to improve the shot feeling imparted by golf balls with ionomer covers, Japanese Patent No. 2709950, for example, proposes a mixed ionomer cover. The mixed ionomer is a mixture of a hard ionomer comprising sodium salt or zinc salt of an olefin-unsaturated carboxylic acid copolymer and a soft ionomer comprising sodium salt or zinc salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylate terpolymer. The mixed ionomer cover makes a golf ball possible to render the shot feeling soft, but impairs the merits inherent to ionomer covers; for example, the repulsion property as well as the scuff resistance (or chunking property) of the golf ball upon a shot lowers.
In recent years, polyurethane is receiving attention as an inexpensive cover material that imparts to golfers a shot feeling analogous to that imparted by the Balata cover and has higher durability than the Balata cover. Japanese Patent No. 2662909, for example, discloses a polyurethane cover formed by curing a urethane prepolymer with a slow-reactive polyamine curing agent.
Japanese Patent No. 2662909 teaches a urethane prepolymer obtained by reacting a polyol such as polyether polyol, polyester polyol or polylactone polyol with a diisocyanate such as 4,4′-diphenylmethane diisocyanate (MDI), 3,3′-dimethyl-4,4′-biphenylene diisocyanate (TODI).
Generally, the soft segment of a polyurethane resin largely influences the performance of the polyurethane resin. The component serving as the soft segment of polyurethane is a polyol forming a urethane prepolymer. The Japanese Patent, however, is completely silent on the relation between a polyol and a polyurethane cover formed using the polyol or a golf ball with the polyurethane cover. Meanwhile, it is likely that golf balls, which are observed to have not so large a difference in performance therebetween when they are each in a brand-new state, come to exhibit a considerable difference in performance therebetween if they are left under a severe condition such as a rainy weather or a very hot weather under a burning sun.